You haven't given that anywhere near enough thought. You can't just slap some zeros onto the start of a v4 address and come up with a bigger address space that

You haven't given that anywhere near enough thought. You can't just slap some zeros onto the start of a v4 address and come up with a bigger address space that somehow works with v4 networking code. The problem isn't backwards compatibility, it's forwards compatibility: v4 isn't forwards compatible with larger address spaces, so it's not possible to do what you're suggesting. There are various ways of making a bigger address space with backwards compatibility to v4 though -- and all of the ways which are actually possible and work are already implemented by v6.